It is well-known to concentrate liquid wastes from LWR's to a limited extent and then to encapsulate such concentrates in various types of matrices such as cement, bitumen or synthetic resin polymers. The waste and matrix mix is then stored in containers. In order to further reduce the quantity of waste and the corresponding number of storage containers, it has been proposed more recently to completely dry the waste and encapsulate a dry product in the matrix material. The techniques being used and developed in an effort to reach a dry product before encapsulation are referred to broadly as "volume reduction".
In this specification, the terms "dried waste" and "dry product" mean waste solids which contain substantially no free water. Combined water, such as water of hydration or crystallization, may be present.
Because of the problems encountered with volume reduction as discussed below, many of the present waste treatment facilities still encapsulate some form of liquid concentrate. This practice leads to a large number of radwaste storage drums which must be stored temporarily above ground and then permanently disposed of, either at sea or by land burial. With present practices for encapsulating concentrate, waste from a 1,000 megawatt (MWe) PWR can lead to the accumulation of more than 2,000 standard drums for each year of normal operation. The waste from BWR's of the same power may require more than 3,000 standard drums per year. Extending such figures to the hundreds of existing and planned nuclear power plants, hundreds of thousands of drums will have to be stored and disposed of each year. For this reason, efforts to develop a satisfactory volume reduction technique have intensified in recent years.
Effective volume reduction can result in considerable savings, both in money and manpower, for a number of reasons. The amount of matrix material needed for encapsulation of a given quantity of waste is reduced where the waste is in dry solid form. Similarly, the quantity of waste that can be placed in each container is increased so that the number of containers necessary is also reduced. Either matrix encapsulation or placement of the waste in containers is considered to be a way of enveloping the waste in a protective envelope for purposes of this specification. As a conservative estimate, the final volume of enveloped waste arising from low level power plant effluents can be reduced by a factor of at least 5 to 15 by volume reduction techniques. A reduction in the number of storage containers produces a corresponding reduction in the capacities of the facilities needed for interim storage, container handling, container transportation, and ultimate disposal and in the manpower required for all such operations. As an example of the estimated cost savings for a 1,000 MWe PWR power station, considering the overall costs for conditioning, encapsulation and ultimate disposal at sea of 1 cubic meter of waste effluent containing 12 weight percent dissolved solids, the savings achievable with effective volume reduction is estimated to be in the range of $500 to $1,500 per cubic meter of effluent. Cost savings in this range will compensate within just a few years for the additional capital investment required for installation of a volume reduction system.
A further advantage of volume reduction is that it enhances safe handling and disposal of the waste material. Since smaller quantities of waste can be handled, stored, transported and ultimately disposed of permanently, there is realized a corresponding reduction in the hazards to personnel and a corresponding increase in the useful life of equipment. Safety to the environment is enhanced both by the smaller number of waste containers and the avoidance of any danger of a releasable water fraction containing radioactive ions.
Notwithstanding the known advantages of volume reduction, a number of difficulties have been encountered in developing an effective volume reduction technique. Attempts have been made to use thin-film evaporators as the drying apparatus in volume reduction systems. However, prior to reaching a dry state, waste concentrate becomes a heavy paste at solids levels in excess of 60 weight percent. This paste dries relatively slowly as a film, and in order to reach dryness in a thin-film evaporator a vacuum is used along with a relatively slow rate of material advancement along the heated surface. Such installations therefore require expensive auxiliary equipment to create the vacuum and the evaporator is not operated at an efficient throughput because the feed rate is limited by the drying rate of the film. In addition, the feed rate must be closely monitored and controlled so that drying occurs at or very near the evaporator outlet. Premature drying will cause blockage of transport passages and jamming of the evaporator rotor. Notwithstanding such control, blockage frequently occurs anyway after relatively short periods of operation due to the gradual buildup of hardened layers of concentrate at the heated wall surface, a condition which is aggravated by the slow rate of material advancement. Therefore, such equipment operates much more efficiently as a concentrator rather than as a dryer.
Other types of dryers have also been proposed for use in volume reduction systems, such as spray dryers and drum dryers. These types of dryers create large amounts of dust particles which are difficult to remove from exiting gas streams and can rapidly erode or jam gas treatment equipment. Spray dryers are further deficient in that solids can buildup in and around the spray nozzles and lead to blockage. Drum dryers are further deficient in that the dry layers formed on the heated drum can be difficult to scrape off or otherwise remove.
Although some of the foregoing problems might be alleviated by incomplete drying of the concentrate, significant moisture content in the waste product leads to problems in the characteristics of the encapsulated product. The presence of water in the radioactive fraction makes it difficult to control the quality of the final waste and matrix product. In other words, the amount of dry cement to be used to make up the final composition depends upon the total water in the waste and matrix mix and the amount of water in wet concentrate or partially dried solids can fluctuate and is difficult to control with any degree of accuracy. As a result, past practices often have led to either too much or too little water in the encapsulated product. Water is also extremely detrimental in a bitumen matrix as this matrix must be heated and the heated matrix causes water vapor to form which interferes with the encapsulating process. Water is also detrimental in most resin polymer matrices as it inhibits the polymerization reaction. For these reasons, the presence of water in the waste fraction results in a product having poor water resistance (because of the presence of non-fixed, leachable radioactive ions), poor chemical resistance, and inferior structural integrity (mechanical strength).